In general, various kinds of mechanical stresses are applied to a photographic material obtained by coating a silver halide emulsion. For instance, negative photographic films are bent when rolled up into a film cartridge or charged in a camera, and they are pulled at the time of frame traveling.
On the other hand, in case of photographic films having a sheet form, such as those for graphic arts, those for direct radiography employed for medical use and so on, it frequently occurs that the photographic films are folded or bent because they are handled directly by the hands.
In addition, great stresses are imposed on all sensitive materials by cutting and processing operations.
When various stresses are applied to a photographic material as described above, the stresses are imposed on silver halide grains through the gelatin medium which is typically the binder for the silver halide grains or through a plastic film which forms a support therefor. The stresses imposed on silver halide grains are known to cause changes in photographic properties of the photographic material. Details of such changes in photographic properties as caused by applied stresses are reported, for example, in K. B. Mather, J. Opt. Soc. Am., Vol. 38, p. 1054 (1948); P. Faelens and P. de Smet, Sci. et Ind. Phot., Vol. 25, No. 5, p. 178 (1954); P. Faelens, J. Phot. Sci., Vol. 2, p. 105 (1954); and so on.
Of these changes in photographic properties, a phenomenon called "stress marks" tends to adversely affect the image quality of the photograph. In the case of X-ray sensitive materials, for example, this phenomenon can involve the danger of erroneous diagnoses.
Therefore, the advent of photographic materials that do not generate stress marks even if the foregoing stresses are applied thereto has been strongly desired.
As means for attempting to solve the above-described problem, some methods as described below have so far been considered.
For example, a method of making it difficult for pressure to reach silver halide grains by incorporating a plasticizer used for polymers or emulsions, or by lessening the silver halide to gelatin ratio in the silver halide emulsion, has been known as the means for improving the pressure-sensitivity characteristics.
More specifically, methods of using, as the plasticizer, a heterocyclic compound in British Patent No. 738,618, an alkyl phthalate in British Patent No. 738,637, an alkyl ester in British Patent No. 738,639, a polyhydric alcohol in U.S. Pat. No. 2,960,404, a carboxyalkyl cellulose in U.S. Pat. No. 3,121,060, paraffin and a carboxylic acid salt in Japanese Patent Application (OPI) No. 5017/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and an alkylacrylate and an organic acid in Japanese Patent Publication No. 28086/78, respectively, have been disclosed.
However, the method of adding such plasticizers as described above is attended with a lowering of the mechanical strength of the resulting emulsion layer and, therefore, there is a practical upper limit to the usable amount of the plasticizer. Furthermore, the method of increasing the content of gelatin is attended by a decrease in the development processing rate. For the above-described reasons and others, none of the foregoing methods can produce completely satisfactory effects. Accordingly, it is desired that the silver grains per se should acquire the property of being resistant to generating stress marks.
However, there are no descriptions of methods for making such an improvement with respect to pressure marks in the foregoing various literatures relating to stress. Also, Japanese Patent Application (OPI) No. 9137/83 and so on have described that desensitization due to applied pressure is caused by using silver iodobromide grains which are produced by growing silver iodobromide grains to have layers of different iodine mol fractions, but no descriptions of improvements in stress marks.
With respect to silver halide grains having a multilayer structure, silver halide grains having a triple-layer structure consisting of AgBr/AgBrI (I=18 or 32 mol %)/AgBr are described in J. Photo. Sci., Vol. 24, p. 198 (1976). Therein, although the matter of the influence which the triple-layer structure has on the developability and the covering power was raised, there is no description of stress marks therein, nor any consideration regarding stress marks, because the silver halide grains used therein were not chemically sensitized.
In addition, silver halide grains having a multilayer structure prepared using an iodine conversion method have been proposed in U.S. Pat. No. 4,210,450. Such grains have an outermost layer having a high iodide content of 95 mol % or more, since iodide conversion is necessarily conducted in the final part of process of preparation. Therefore, such grains are undesirable in terms of stress marks.
Moreover, a highly sensitive photographic emulsion containing silver halide grains constructed by at least three silver iodobromide phases differing in iodide content, in which the overall iodide content is at least 12 mol %, and a preparation method therefor are described in Japanese Patent Application (OPI) No. 181037/83. According to the preparation method proposed by said patent application, however, homogeneous iodide distribution in each phase cannot be achieved. Also, no description with respect to the problem of stress marks is presented therein.
On the other hand, enhancement of developing activity and increases in sensitivity have been aimed at by using laminate type silver halide grains in which two or more shells are laminated on the surface of a core, as described in Japanese Patent Application (OPI) No. 22408/78, Japanese Patent Publication No. 13162/68, J. Photo. Sci., Vol. 24, p. 198 (1976), and so on.
However, the silver halide grains prepared for the above-described aims do not always improve the stress characteristics, and the stress mark problem still remains largely unsolved. For example, multilayered silver halide grains consisting of pure silver bromide (core)/silver iodobromide (iodide content: 1 mol %)/pure silver bromide are described in Japanese Patent Application (OPI) No. 22408/78. However, those grains generate intense pressure marks, and thus from the viewpoint of pressure characteristic they have a problem analogous to that of conventional silver iodobromide emulsions having completely homogeneous iodine distribution.
Furthermore, it is noted that the pressure marks show up as photographic fog in a negative silver halide emulsion, whereas in a positive silver halide photograph they show themselves in the lowering of optical density (because the fog generates inside the grains).